1. Field of the Invention
The present invention relates to a scroll type compressor used in, for example, a refrigerating cycle apparatus, wherein compression chambers are formed between a stationary scroll and a revolving scroll, a refrigerant gas is sucked and compressed in the compression chambers, and the gas is discharged.
2. Description of the Related Art
Recently, scroll type compressors, among refrigerant gas compressors, have widely been used in refrigerating cycle apparatuses.
The scroll type compressor can perform a compression function with a higher efficiency than, for example, a rotary type compressor, and a valve mechanism is not required. Thus, the number of parts can be reduced, and operation noise can be decreased.
In the scroll type compressor, a rotary shaft is contained within a sealed casing, and a scroll compression mechanism for sucking and compressing a refrigerant gas is provided at one end portion of the rotary shaft.
The scroll type compression mechanism comprises a combination of a revolving scroll engaged with an eccentric portion formed integral to the rotary shaft, and a stationary scroll fixed on a support frame. The revolving scroll revolves, without rotating about its own axis.
Each of the revolving scroll and stationary scroll comprises a plate-like spiral blade portion and a disc portion (generally termed "mirror plate") formed integral to one end portion of the blade portion.
The blade portions of the revolving scroll and stationary scroll are engaged with one another, thereby defining a pair of compression chambers or compression spaces between the disc portions.
In accordance with revolution of the revolving scroll, a refrigerant gas is sucked into the peripheral compression chambers.
The volume of each chamber is gradually reduced, while shifting to the center of the spiral.
When the chambers reach the center of the spiral, the gas is compressed to a predetermined high pressure and is discharged from a discharge port facing the center of the spiral.
The problem of the above-described scroll type compressor is as follows.
The compression ratio of a regular, e.g. rotary type compressor is automatically adjusted to an optimal condition constantly in accordance with operation conditions.
By contrast, the scroll type compressor is driven at a constant compression ratio, irrespective of a variation in loads such as discharge pressure and suction pressure of the refrigerating cycle.
Under the operation conditions in which the compression ratio is too large or too small, the compression loss is high and the performance lowers.
For example, in the case where the suction pressure is high and the compression ratio is very small, the gas pressure in the compression chambers becomes extremely high and the stress on the blades of the scrolls and associated parts increases. As a result, the reliability of the compressor is degraded.
This problem can be solved by providing a so-called release mechanism which returns part of compressed gas in the compression chambers directly to a gas suction unit, thus reducing the gas pressure in the compression chambers.
A feature of the scroll compression mechanism, however, is that a pair of compression chambers are formed symmetrically. These chambers suck and compress gas simultaneously.
Thus, it is thought that two release mechanisms are provided for the respective compression chambers and the same amount of gas is released simultaneously from the respective chambers.
Only a slight difference in amount of released gas causes a pressure difference between the compression chambers, and the revolving scroll may revolve with an inclination.
Consequently, part of the revolving scroll is put in pressured contact with the stationary scroll, abrasion or damage may occur.
Thus, the release mechanism must have a relatively complex structure, and a very difficult adjustment is required to exactly release the same amount of gas from the two compression chambers.
A technique for solving this problem is disclosed in Japanese Patent Disclosure No. 63-259104. This application discloses a scroll type compressor wherein a passage for communication between a gas suction unit and a compression space is formed in a stationary scroll, and this passage is provided with a volume control valve.
Another technique is disclosed in Japanese Utility Model Application No. 62-105389. This application discloses that a pair of by-pass passages are formed in the blade-side bottom surface of a disc portion of a stationary scroll, and a communication path for connecting these by-pass passages is formed in the surface of the stationary scroll opposite to the blade-side bottom surface. The respective by-pass passages are provided with actuators for opening and closing the by-pass passages.
These techniques, however, increase the number of parts and the manufacturing cost, and it is difficult to exactly release the same amount of gas from the equal-pressure compression chambers. Thus, the reliability of control is low.